1. Field of the Invention
This present invention relates to multiple-color image processing using inks or dry toners and, more specifically, to controlling the amount of ink or dry toner used in forming the image on a receiver.
2. Description Relative to the Prior Art
Conventional electrophotographic process steps employed within image processing equipment are known to employ light-emitting and light detection devices to measure of the amount of toner used in image formation (e.g., U.S. Pat. Nos. 5,325,153; 5,583,644; 5,842,080; and 6,055,011). Similarly, conventional printing processes used in image processing equipment are known to employ light-emitting and detection devices to measure the amount of inks used in the image formation (e.g., U.S. Pat. No. 5,854,680). The foregoing prior art references utilize light-emitting diodes (LEDs) in conjunction with photocells to quantify the amount of dry toner or ink used by measuring the amount of light reflected by image test patches. Specific attention is given to the very different light absorption and reflection characteristics of black vs. colored toner when illuminated with near infrared light. The use of other light sources, such as colored LEDs, is considered inferior due to higher cost and increased complexity in implementation (U.S. Pat. No. 6,055,011). Furthermore, in using reflected light to measure the quantity of dry toner or ink, the optical properties of the surface underneath have to be taken into consideration. These considerations might include the manufacture of intermediate transfer rollers with certain preferred optical characteristics (U.S. Pat. No. 5,842,080), switching of the illumination intensity to improve the signal-to-noise ratio (U.S. Pat. No. 5,325,153), or arranging image sample patches in support of a preferred measurement procedure (U.S. Pat. No. 5,854,680) and sequence.
Other disclosures within the prior art teach the use of light transmission through image test patches to measure the amount of colorant used in the image formation process. Such arrangements are described in JP-B 4-18310 (as referenced in U.S. Pat. No. 5,842,080, the original was not available to the authors) and in U.S. Pat. No. 5,903,800.
The previously discussed prior art disclosures measure the amount of toner used in the image formation process as an integral part of the image processing apparatus. Such measurements are used to derive control signals for the purpose of automatically adjusting the operating setpoints of the printing process itself so that printing quality is maintained and consistent over a large range of operating conditions and over long periods of print production.
It should be apparent from the foregoing discussion that there remains a need within the art for an apparatus that can control the amount of colorants used in image formation. It is desirable that amount of colorants used remain constant despite changes in (a) print productivity, (b) environmental condition, (c) lifetime fatigue of consumables, or (d) malfunctioning of components. It will be readily apparent that the last two areas can be somewhat controlled and considered in the design and manufacturing process of the image processing equipment. Environmental conditions and productivity of the image processing apparatus in the customer site are beyond the control of the manufacturer.
Accordingly, there exists the need within the art for image processing apparatus that have the capability of adjusting operating parameters automatically, such that image quality produced on the receiver is, at all times, within the manufacturer""s stated specifications and stable over extended periods of time.
The present invention addresses the previously discussed problems within the prior art by controlling the image processing apparatus to ensure that the amount of colorants used in the image formation remains constant despite changes in lifetime fatigue of consumables and malfunctioning of components. The hardware components and methods according to this invention are particularly suited to be implemented in conjunction with a multicolor printing process utilizing an endless loop for either the purpose of recording the image or transporting the image receiver.
According to one aspect of the present invention, light-emitting and light-detecting devices are mounted in a facing relationship such that they are, respectively, on the inside and the outside of the endless loop. The light-emitting device will emit light through a test patch on the receiver into the light-detecting device at various levels, depending on the density of colorant used in the test patch or within the image on the receiver. Furthermore, the light-emitting devices employed by the present invention are selected to emit colored light of a wavelength within the absorption spectrum of the colorants used in the printing process that is roughly complementary in color to the colorants used in the patch to be tested. The image processing apparatus creates image test patches that are arranged such that light, complimentary to the colorant forming the image test patch, is transmitted through test patch and received by the light-detecting device prior to being used in making measurements. The measurements made in this fashion are superior to those made by prior art devices. The light-emitting and the light-detecting device for each of the colors amount of the transmitted light that is absorbed increases, yielding qualitatively similar characteristics for all four colorants in contrast to prior art. Accordingly, the detecting devices for all four colorants show a similar exponential loss in intensity as the density of the colorant increases. This similar data received for each colorant is processed identically in contradistinction to the prior art techniques shown in U.S. Patent Nos. 6,055,011 and 5,325,153.
The selection of LEDs that are roughly complimentary to the colorants maximizes the signal for each colorant and, thus, also allows the measurement of colorant density on the image receiver after transfer of the image. Observations have shown that infrared illumination of colorants through the receiver (such as paper) produces measurable signals proportional to the amount of black colorant. The other three colorants are indistinguishable from each other and yield no measurable signal as a result of infrared stimulation to an image on most paper-like receivers.
The present invention provides a system that uses of the same detection circuit for each of the colors with the only difference being the color of the LED used for each of the specific colors at the color sensing point. The use of an identical circuit is a desirable feature of the present invention because it reduces the manufacturing cost of the multicolor image processing apparatus. Also, in accordance with another aspect of the present invention, the absorption characteristics for all colorants will be the same by employing the same detection circuits normalized to a specific color. Additionally, a similar mechanical arrangement is used to illuminate the receiver at each of the color sensing points.
Furthermore and, in accordance with the present invention, the light-emitting devices for each of the colorants is modified to an individualized fixed and predetermined aperture. Furthermore, the light-emitting devices for each of the colorants includes the provision of allowing the adjustment of the maximum light output by adjusting the electrical current that flows through the light emitting diode (LED) used as the light-emitting device. According to the present invention, the optical aperture and maximum light output of the illumination are selected to produce a signal of similar amplitude in the light detection circuit for each of the sensing points. By making these modifications to the illumination, the absolute value and the dynamic range of the electrical signal that is derived from and is proportional to the detected light are essentially equal and independent of colorant. The light detection circuit typically includes circuitry to perform a logarithm operation on the input signal. Such an operation can be performed by a special analog device such as a Logarithmic Amplifier (e.g., Burr-Brown Log100JP), or digital devices that can be programmed to provide this function (such as microprocessors programmed accordingly to perform the logarithmic function).
It should be noted that the modifications to the illumination, in accordance with this disclosure, produce voltages similar in magnitude for similar densities of colorants.
It is another aspect of this invention to describe details of the electronic circuit of the light detection circuitry intended to protect the measuring circuitry incorporated in typical image processing equipment.
It is another aspect of this invention that the light intensity regulation provided according to the above is used in the operation and calibration of this device. The methods described for operation of the device and calibration of the device is incorporated into the controller of the image processing apparatus. The seamless integration of these methods into software governing the print production mode and the service mode yield the desired consistency in print quality over an extended range of operating conditions.